In this post, let’s have a look at the error message “5.1.0 – Sender denied” that you get when working with Microsoft 365 Exchange Online.
These are the errors that you usually receive when there is a problem in delivering the email message that was sent by you. These errors are generated and sent by Office 365. This is usually alled as DSN which stands for delivery status notification or in simple words bounce messages.
NDR or Non-delivery report is one of the common type of message that you receive which indicates to the user that the email was not delivered.
5.1.0 – Sender denied
A common cause of this NDR is when you use Microsoft Outlook to save an email message as a file, and then someone opened the message offline and replied to it. The message property only preserves the legacyExchangeDN attribute when Outlook delivers the message, and therefore the lookup could fail.
Either the recipient address is incorrectly formatted, or the recipient couldn’t be correctly resolved. The first step in resolving this error is to check the recipient address, and send the message again.
For more information, see Fix email delivery issues for error code 5.1.0 in Exchange Online.